1. Field of the Invention
The present invention relates to a display control apparatus and, more particularly, to a display control of a display apparatus having a memory performance such as a ferroelectric liquid crystal (hereinafter, referred to as FLC) display apparatus or the like.
2. Related Background Art
In a computer display apparatus, the realization of a large screen and a high fineness is being progressed. Such a tendency is largely concerned with a fact that GUI (graphical user interface) is generally used as a user interface of a computer system.
Under the multiwindow environment of the GUI, it is preferable to use a display having a degree of fineness of at least 1000.times.1000 or more and a size of 15 inches or larger.
In the field of a flat panel display, the realization of a large screen and a high fineness is also similarly demanded.
There are at present several systems of flat panel display.
For example, there are a high time division driving system of a twisted nematic liquid crystal (STN), a system for a black and white display (NTN) and a plasma display system as modifications of the STN, and the like. Each of those systems has the same image data transfer method as that of the CRT. As a screen updating method, each of those systems uses a non-interlace method of a frame frequency of 60 Hz or higher. Therefore, the total number of scanning lines constructing one screen is equal to about 400 to 480. A flat panel display of a large size such that the total number of scanning lines constructing one screen is equal to or larger than 1000 is not yet obtained.
This is because since the above displays don't have a memory performance in terms of the driving principle, a refresh cycle of a frame frequency of 60 Hz or higher is necessary to prevent a flickering, so that there is a problem such that one horizontal scanning time is a short time of 10 to 50 .mu.sec or less and a good contrast cannot be obtained.
A display apparatus using an FLC which can solve the above problem has been proposed.
The FLC has a "memory performance" as one of the characteristic features. The memory performance denotes that the liquid crystal cell holds a display state which was changed by applying a voltage. Such an FLC and an FLC display apparatus have been disclosed in detail in, for example, the Official Gazette of U.S. patent Ser. No. 4,964,699 by Inoue.
The display apparatus using the FLC can realize a display of a large screen and a high fineness because of its memory performance.
However, when a degree of fineness is high, the frame frequency is low. Therefore, the multi interlace (skip scan of a plurality of scanning lines) is needed to prevent the flickering. On the other hand, a partially rewriting scan (only the scanning lines in a rewriting area are scanned) method is necessary to have an enough display response speed as a computer display.
The partially rewriting scanning method has been clearly described in, for example, the Official Gazette of U.S. patent Serial No. 4,655,561 by Kanbe, et al.
Particularly, in the FLC display apparatus, the above partially rewriting scanning method is suitable for a moving display of a mouse, cursor, or the like, a scroll display of a multi window, or the like. The partially rewriting scan of two different areas cannot be executed in the same time, however, in case of a system such that the partially rewriting scan is executed by designating a start address and an end address of the partially rewriting scan, there is a problem such that the moving display of the mouse, cursor, or the like cannot be executed during the scroll display of the multi window.
For example, the scroll display of the window and the display of a pointing device will now be considered and their movements will now be presumed. First, a partially rewriting scanning request of the window scroll display is generated. The scroll partially rewriting scan is started for the display panel and, thereafter, the pointing device moves. In this state, however, the rewriting scan of the pointing cannot be executed until the completion of the scan of the final scanning line address of the window. Therefore, the pointing device discontinuously moves in accordance with the size (the number of partially rewriting scanning lines) of the window, so that there is a problem such that the moving display becomes obviously unnatural.
In the conventional partially rewriting scan, the updated area is displayed by only the non-interlace.
Therefore, when the partially rewriting area is large, a flickering occurs at the time of the partially rewriting scan because of a low frame frequency of the FLC display apparatus.
Particularly, when such a partially rewriting operations are continuously executed, an unpleasant feeling is given to the user and can result in a cause of a physical disorder by the VDT. Generally, when the non-interlace scan is performed at a frame frequency of 40 Hz or less, a flickering occurs.
As characteristics of the FLC, there is a problem such that when the partially rewriting scan is repeatedly executed, in case of an insufficient memory performance of the display panel, the contrast of the area in which the partially rewriting operation is not executed gradually deteriorates or the like.